Pain is a serious medical problem. While it is well known that hyperexcitability of dorsal root ganglion (DRG) sensory neurons can contribute to neuropathic and inflammatory pain, the cellular and molecular changes that underlie this hyperexcitability are not fully understood. This lack of knowledge has hindered the development of better therapeutics. Studies indicate that sodium channel properties are altered by inflammation and nerve injury. Changes in sodium currents can substantially alter the excitability of DRG neurons. We have found that mutations that cause paroxysmal extreme pain disorder (PEPD) can significantly increase resurgent sodium currents produced by Nav1.7 in DRG neurons. Furthermore, we have exciting new data indicating that a spinal cord injury that causes increased pain can significantly increase resurgent currents in DRG neurons. We propose that resurgent sodium currents contribute to spontaneous firing, hyperexcitability and the initiation of pain sensations in DRG sensory neurons. Unfortunately, our understanding of the molecular mechanisms that underlie resurgent currents, especially in DRG neurons, is poor. We have developed a neuronal expression system for recombinant voltage-gated sodium channels that uniquely positions us to investigate these currents. In this project we will: 1) Establish the molecular determinants of resurgent currents in DRG neurons. 2) Determine how resurgent currents are regulated by phosphorylation, inflammatory mediators, reactive oxygen species, acidity, and other modulators that impede fast-inactivation of sodium channels and contribute to enhanced pain sensations. 3) Determine if resurgent currents are sensitive to modulation by local anesthetics, anti-convulsants and other agents that target voltage-gated sodium channels. 4) Investigate the consequences of resurgent currents on sensory neuronal excitability. Identifying the molecular mechanisms underlying resurgent current generation will enhance our ability to identify the roles of these currents in pain and other disorders of excitability, and to develop therapeutic strategies specifically targeting resurgent currents.